Feb 06 2019

Progeny Mk6 Block II Flight 2 Analysis

Ever since the Block I outperformed all expectations and flew into the region of space we expected to use the Block II to reach, this rocket hasn’t been able to find a good use in our launch lineup which is why this is only the second launch since it was first conceived back in late 2017. The mission however was well-suited to our most powerful rocket to date, which was to go as far out into space as possible to gather new radiation data and look for a second radiation belt. In addition to building on the previous radiation discoveries, an additional mission goal was to test new hibernation technology to extend the vessel’s battery life long enough to last a mission that could take several hours.

The Flight

The rocket had a clean launch, leaving the mounting base without impacting the support rail while firing on all 5 solid rocket motors for a total initial thrust output of 214kN, producing just over 6Gs of force to keep it pointed downrange as the nose began to lower and the fixed fin angles began to spin the rocket up. The main difference between the Block II and Block I are the 4 radial boosters that accelerate the larger and heavier Block II rocket up to speeds comparable to a Block I. They burn from 5-6 seconds before being discarded. During the first flight, the booster separation led to an impact with two of the large lower fins thanks to the decouplers throwing them away with enough force to remain lateral to the rocket. The solution was simply to let them drop away on their own by force of the rocket’s speed and spin. All four boosters made a clean separation although the rocket exhaust did scatter them and actually flung one back far enough to impact the Ascension service towers! Thankfully no major damage was done.

Once through the radial booster stage the Block II flies very similar to the Block I in both altitude, speed and sequence of staging events. The next major event is  MECO-1 when the lower core solid rocket booster runs out of fuel. This occurred at L+34s 16.6km ASL followed by separation 1 second later. Here is where another fault of the first launch was identified, when the second stage engine was suspected to have scraped past its shroud upon decoupling. We had the shroud removed for this launch to avoid this possibility.

After a coast period of 12 seconds the rocket had pitched over more than 1.5° and triggered the AFCS to light off the second stage booster at 24.4km, which put out just over 15kN of force at that altitude, enough to keep the rocket accelerating as it flew further upwards at speeds exceeding 800m/s. The booster burned for 13 seconds before MECO-2, followed a second later by stage separation, followed a second after that by ignition of the liquid propellant engine.

Here is another deviation from the Block I as the Block II carries twice the fuel in its third stage for a final engine burn that lasts just over one minute, which began at 35.2km and pushed the rocket the rest of the way out of the atmosphere. When MECO-3 arrived at L+2m1s the rocket was traveling at 2.794km/s heading for an apokee of 3,123.563km!

While the height was an impressive new record, it also meant it would take the rocket over 2 hours to climb that high and fall back down to the atmosphere. The batteries on board, at the current power draw, would run out in a little over 32 minutes. To make sure the power would last throughout the mission the AFCS had been redesigned to allow the probe core to enter a state of hibernation. The instructions would be saved to disk, the antenna would be switch off and the probe core would then be switched off. A separate mechanical device sipping just a watt of energy would monitor a timer and switch the probe core back on after almost a minute. The probe will reboot the CPU and the instructions will be reloaded and executed. If the rocket was still in comm range it would switch on the antenna and fire back a report, otherwise to further conserve power the antenna would remain off and the latest telemetry data would be stored on disk to be beamed down once comms were restored.

Initial mission planning attempted to use the Launch Vehicle Designer from ArrowstarTech but we could not get the ascent trajectory to match up with what was observed on the first flight, largely because the rocket is unguided and spinning. This leads to oscillations of the nose which causes extra drag not to mention the spin of the rocket itself removes energy and so we were getting results as high as nearly 5Mm for an apokee. Although mission planners were fairly sure that was way too high, out of caution we ensured that if such a height was reached enough power would remain for the entirety of the mission and that is how we settled on data checks every one minute.

By the time the rocket fell back below 1Mm for a chance to contact the ground at L+2h14m36s, Kerbin had rotated underneath it to present the Great Desert far to the west of KSC. The space center had spun out of sight by now but the tracking dishes at the Arekibo Radio Observatory were able to pick up its signal and track it all the way down to just before atmospheric entry when terrain of the crater ring blocked the signal at L+2h21m18s. All the data it collected while it was out of contact however was safely downlinked before then. Approaching the atmosphere, the rocket was once again laid out in a flat spin similar to how it returned from the first mission. This means there is a chance it could have survived re-entry in a similar manner. If any science expeditions head out to the area in the future we will ask them to look, but to just retrieve the payload would be prohibitively expensive compared to simply buying new parts.

Flight telemetry data.

Flight Analysis

Second Stage Flight

We’ve recently adopted a new calculation for Angle of Attack, so we are unable to compare this flight as extensively with previous flights like we did before, but at least we have the recent Block I launch at the start of the new year to use:

The timeline covers coast and boost – we have no idea what caused the relatively large oscillation during the engine burn, perhaps some wind shear. Aside from that anomaly though the initial stability of the rocket comes rather close to the Block I. Still, despite this the rocket was laid out flat upon return while the Block I manages to stay upright and come down engine first as designed. Why this remains the case is likely to due with how much taller the Block II is, but we’ll need more flight data to determine that. We will continue to leave the engine shroud off for future launches.

Probe Hibernation

Since we didn’t climb as high as the LVD predicted, when the rocket lost contact over the terrain on the way down it still had 63.5% of its battery power left. The new hibernation capability was a complete success and collected all the data at 1-minute intervals like it was programmed to. The design is very flexible and will allow us to apply it to future orbital missions as well to allow probes to stay up longer in the period before RTGs are allowed to remain in space for extended amounts of time.

Radiation Discoveries

Not only did this mission confirm the radiation that was detected over the poles extends into a second belt around Kerbin, it went far enough to breach the magnetosphere, which is the protective bubble that surrounds the planet and blocks out space radiation.

Much like the radiation belts, the magnetosphere’s shape is not really a perfect sphere but it instead made oblate and stretched out by the kerbolar wind streaming against it. How far it stretches or tails off (hence the term ‘magnetotail’) is still unknown but it could possibly reach the orbit of Mun. The difference in radiation inside and outside of the magnetosphere was also not as great as scientists had predicted, although everyone is cautious to note we did not travel too far beyond the magnetopause and levels could begin to rise after a certain distance. Still, our first true taste of inter-planetary space was a pleasant surprise!

Future Plans

This was the final flight of the Mk6 Block II. The recent Ascension mission made it clear that it is time to move on to the Mk7, which was last discussed just over a year ago when announcing the transition from the Mk5 to the Mk6. We now have additional goals for the Mk7, and although we will release more information on that later this month you may pick up some hints in the Ascension flight analysis being published next week.